Deserialize errorType from extensions

The errorType field on GraphQLError is specific to GraphQL Java and when
serialized via GraphqlErrorHelper#toSpecification it becomes an
extension with the key "classification".

This commit ensures that we correctly deserialize back to errorType from
the "classification" extension but unfortunately that's limited to the
ErrorClassification enums that we know of.

samples/webmvc-http-security/src/test/java/io/spring/sample/graphql/SampleApplicationTests.java

@@ -51,8 +51,7 @@ class SampleApplicationTests {
 				.errors()
 				.satisfy(errors -> {
 					assertThat(errors).hasSize(1);
-					assertThat(errors.get(0).getExtensions().get("classification"))
-							.isEqualTo(ErrorType.UNAUTHORIZED.name());
+					assertThat(errors.get(0).getErrorType()).isEqualTo(ErrorType.UNAUTHORIZED);
 				});
 	}
 
@@ -71,8 +70,7 @@ class SampleApplicationTests {
 				.errors()
 				.satisfy(errors -> {
 					assertThat(errors).hasSize(1);
-					assertThat(errors.get(0).getExtensions().get("classification"))
-							.isEqualTo(ErrorType.FORBIDDEN.name());
+					assertThat(errors.get(0).getErrorType()).isEqualTo(ErrorType.FORBIDDEN);
 				});
 	}
 
@@ -103,8 +101,7 @@ class SampleApplicationTests {
 				.errors()
 				.satisfy(errors -> {
 					assertThat(errors).hasSize(1);
-					assertThat(errors.get(0).getExtensions().get("classification"))
-							.isEqualTo(ErrorType.UNAUTHORIZED.name());
+					assertThat(errors.get(0).getErrorType()).isEqualTo(ErrorType.UNAUTHORIZED);
 				});
 	}